Concurrent Servers

1
Concurrent Servers
2
Idea Behind Concurrent Servers
Server
X
Client 1
Server 1
3
Idea Behind Concurrent Servers
Server
Client 1
Server 1
4
Idea Behind Concurrent Servers
Server
Client 1
Server 1
Client 2
5
Idea Behind Concurrent Servers
Server
Client 1
Server 1
Client 2
Server 2
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Idea Behind Concurrent Servers
Server
Client 1
Server 1
Client 2
Server 2
7
Idea Behind Concurrent Servers
Server
X
Client 1
Server 1
Client 2
Server 2
8
Idea Behind Concurrent Servers
Server
Client 1
Server 1
Client 2
Server 2
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Creating a New Server - fork()

• listenfd Socket( )
• Initialize server address
• Bind( listenfd, )
• for ( )
• / wait for client connection /
• connfd Accept(listenfd,)
• if( (pid Fork() ) 0)
• /I am the child /
• Close(listenfd)
• service_client(connfd)
• Close(connfd)
• exit(0)
• else / I am the parent /
• Close(connfd)

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Points to Note

• fork() is called once
• but it returns twice!!
• Once in the parent server and
• Once in the child server
• How to distinguish parent and child??

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Points to Note

• fork() is called once
• but it returns twice!!
• Once in the parent server and
• Once in the child server
• How to distinguish parent and child??
• Return value in child 0
• Return value in parent process id of child

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Points to Note

• fork() is called once
• but it returns twice!!
• Once in the parent server and
• Once in the child server
• How to distinguish parent and child??
• Return value in child 0
• Return value in parent process id of child
• Child server exits after servicing the client.

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Running another program in child exec()
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Running another program in child exec()
Inetd daemon
listenfd Socket() Connfd Accept()
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Running another program in child exec()
Inetd child
Inetd daemon
Close(listenfd)
listenfd Socket() Connfd Accept()
Fork()
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Running another program in child exec()
Inetd child
Inetd daemon
Close(listenfd)
listenfd Socket() Connfd Accept()
Fork()
Exec()
Service telnet client Close(connfd)
Telnet server
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Different Types of exec()

• int execl(char pathname, char arg0, , (char
  )0)
• int execv(char pathname, char argv)
• int execle(char pathname, char arg0, ,
  (char )0, char envp)
• int execve(char pathname, char argv, char
  envp)
• int execlp(char filename, char arg0, ,
  (char )0)
• int execvp(char filename, char argv)

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Properties of exec()

• Replaces current process image with new program
  image.
• E.g. inetd image replaced by telnet image
• All descriptors open before exec remain open
  after exec.

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Getting IP address/port from socket

• int getsockname(int sockfd, struct sockaddr
  localaddr, socklen_t addrlen)
• Get the local IP/port bound to socket
• int getpeername(int sockfd, struct sockaddr
  remoteaddr, socklen_t addrlen)
• Get the IP/port of remote endpoint
• Why do we need these?

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Two other useful functions

• struct hostent gethostbyaddr (void addr, size_t
  len, int type)
• Converts from IP addr to domain name
• struct hostent gethostbyname (char name)
• Converts from domain name to IP address
• struct hostent
• char h_name/ official name of host /
• char h_aliases / alias list /
• int h_addrtype / address type /
• int h_length / address length/
• char h_addr_list / address list /

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Signals

• Signal is a notification to process (from OS or
  from another process) that an event has occurred.
• Type of event determined by type of signal
• Try listing all signal types using
• kill l
• Some interesting signals
• SIGCHLD, SIGTERM, SIGKILL, SIGSTOP

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Handling Signals

• Signals can be caught i.e. an action associated
  with them
• SIGKILL and SIGSTOP cannot be caught.
• Actions can be customized using
• sigaction()
• which associates a signal handler with the
  signal.
• Details in page 120 of Stevens book
• Default action for most signals is to terminate
  the process
• SIGCHLD and SIGURG are ignored by default.
• Unwanted signals can be ignored
• except SIGKILL or SIGSTOP

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Zombie Processes

• When a child server dies, a SIGCHLD is sent to
  the parent server.
• If parent doesnt wait()on the child, child
  becomes a zombie (status Z seen with ps).
• Zombies hang around forever.

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How to avoid zombies?

• Parent should install a signal handler for
  SIGCHLD
• Call wait()/waitpid()inside the signal handler
• void handle_sigchld(int signo)
• pid_t pid
• int stat
• pid wait(stat)
• printf(child d terminated\n, pid)